In recent years, as next-generation power semiconductor materials, materials such as silicon carbide (SiC), gallium nitride (GaN), diamond, sapphire (Al2O3) and aluminum nitride (AlN), which are wide band-gap semiconductors, have been paid attention to. For example, silicon carbide (SiC) has excellent physical property values such as a band gap 3 times and a dielectric breakdown electric field strength about 7 times those of Si semiconductors, and is superior in high-temperature operability to current silicon semiconductors and excellent in terms of its small size and high energy saving effect as well. Further sapphire wafers are exhibiting increasing importance as electronic devices having optical elements, for example, components for high-performance overhead projectors, due to their chemical stability, optical property (transparency), mechanical strength, thermal property (good heat conductivity) and the like. The diameter enlargement and the mass production of substrates are advanced toward the real spread of these next-generation power devices, and along therewith, the importance of the substrate processing technology is also increasing. In the processing, as in Si, a circular cylindrical single crystal (ingot) to be used for wafers is sliced and thereby cut out into a circular disc shape. Then, the surface of the sliced circular disc-shape single crystal is to be flattened, but first, lapping processing in order to roughly remove the roughness of its surface is carried out by using a lapping plate. Thereafter, polishing processing in order to further improve the flatness of the surface of the circular disc-shape single crystal and remove micro flaws on its surface to thereby make a mirror surface is carried out. Therefore, making the flatness of a circular disc-shape single crystal surface to be enhanced and micro flaws to be few by lapping processing is important in order to impart an influence to polishing processing thereafter.
Since SiC is by far harder than Si and is also a material chemically and thermally stable, the employment of the usual abrasive grains to be used for lapping processing of Si and the wet etching to be used for removal of damaged layers of Si as a workpiece, for processing of SiC, is conventionally difficult. Hence, the processing of SiC cannot help relying on the chemical mechanical polishing (CMP), and the processing of SiC resultantly takes a long time and decreases the productivity. Further sapphire also has a modified Mohs hardness next to diamond and silicon carbide and a high resistance to chemicals, and is very difficult to process.
Then, for lapping processing of high-hardness, brittle, difficult-to-grind materials such as SiC, a lapping plate high in hardness such as a cast iron or a ceramic substrate is used. In recent years, in order to make the flatness of a circular disc-shape single crystal surface to be enhanced and the micro flaws to be few, there is also employed the lapping processing (hereinafter, also referred to as “diamond lapping”) in which a metal-based plate of copper, a resin-coated copper, tin or the like is used and the plate and diamond abrasive grains are combined. Since the plate of copper, a resin-coated copper, tin or the like is softer than that of cast iron or the like, and even if the diamond abrasive grains are used in the state of being free abrasive grains, the abrasive grains are embedded in the plate surface, the abrasive grains exhibit an action and effect similar to those of fixed abrasive grains (see, for example, Patent Document 1). In order to shorten the time for abrasive grains to be embedded in a plate surface, there is further disclosed means to embed the abrasive grains in the plate surface by imparting an ultrasonic vibration to a pressing member (see, for example, Patent Documents 2 and 3).